2012 Atlantic Hurricane Season Birdseye Discussion #74

By: NCHurricane2009 , 9:55 AM GMT on August 09, 2012

...AUGUST 9 2012...6:10 AM EDT...
Ernesto crossing the Bay of Campeche waters...expected to re-strengthen into a hurricane shortly...and make its final landfall into mainland Mexico later this afternoon.

Elsewhere...Florence's remnant still being watched for re-genesis into a tropical cyclone...and tropical wave Invest 92-L is likely to become the next Atlantic tropical cyclone.

See all three special feature sections for further details on Ernesto...Florence...and Invest 92-L.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1933Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.


This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

Since previous discussion #73 (8:01 PM EDT Aug 7)...Ernesto strengthened to 85 mph winds for its Yucatan landfall...weakened only marginally to 45 mph max winds during the afternoon of August 8...and has quickly regained strength while crossing the Bay of Campeche waters this early morning. Favorable conditions of warm waters...low shear...and grand anticyclonic upper outflow continue to persist with this sytem...and therefore the NHC expects Ernesto to regain hurricane strength before its final landfall into mainland Mexico expected this afternoon. As of 1 AM CDT this morning...I am predicting this afternoon's landfall intensity to be 80 mph max winds (as shown in Figure 1 below). This is slightly below the 85 mph peak Ernesto had for its first Yucatan landfall...because I see a ragged core of t-storms on infrared satellite rather than a tightly-wound-up organzied core.

Figure 1: Tropical Storm Ernesto forecast graphic created early this morning.

Track forecasting has been more challenging with Ernesto. Just after I released my previous forecast in discussion #73 (which had a north bias compared to NHC's)...Ernesto suddenly turned straight west and made landfall in the Yucatan further south than expected. Low-level steering solution in discussion #73 cited that Atlantic surface ridge (paragraph P5 feature) was going to re-build...but I had failed to interpret this would translate to a more west track as opposed to a continued WNW track.

I also cited (in discussion #73 low-level steering solution) that a low-level ridge weakness would redevelop from paragraph P1 upper trough/surface frontal system...growing over the eastern US by 11 PM Thursday (tonight)...then growing into the Gulf of Mexico by 11 PM Fri thru 11 PM Sat. At the time I used that forecasted ridge weakness to suggest a more northward and slower track compared to NHC's. The more westerly-than-expected track of Ernesto has made him out-of-phase to feel any influence from this ridge weakness...I may have also considered this ridge weakness too much...and the models also show a less imperssive low-level ridge weakness...now keeping the Atlantic surface ridge (paragraph P5) linked with central US surface ridge (paragraph P1). Evidence that the central US surface ridge is re-building is also described in paragraph P1. In this forecast (shown in Figure 1 above)...I simply agree with the NHC forecast this time around...and also agree with the WSW bend shown after landfall...which would occur if the central US low-level ridge continues re-building to Erensto's north.

Florence's remnant low has been downgraded to a remnant tropical wave tracking WNW...passing just NE of the Lesser Antilles. It is still directly under the paragraph P6 tropical upper ridge axis for better upper outflow...allowing the remnants to continue firing t-storms.

Judging by above thermo birdseye chart...Florence has exited the dry air region that caused her to dissipate in the first place (source of dry air in paragraph P5). The next obstacle noted for Florence's remnant continues to be southwesterly vertical shear due to mid-ocean upper vorticity (paragraph P3). As of this writing...we are roughly at the 48-hour point in the 200-mb outlook released in discussion #73...and this outlook is important as it showed an aggressive break-up of this upper vorticity that favors Florence's regeneration. I must say...the current 200-mb wind barbs in above atmo chart match fairly well to this 200-mb outlook...so I still think Florence has a chance to regenerate. If the forecasted break-up of mid-ocean upper vorticity begins falling behind schedule...then southwesterly shear would indeed stop Florence from regenerating. Due to the southwesterly shear potential...the NHC cites a 0% chance of regeneration in next 48 hours.

Track-wise...discussion #72 forecast for Florence's remnant suggested a more WNW track longer-term due to erosion of low-level Atlantic ridge from paragraph P1 upper trough/surface frontal system...and this gradual WNW turn is occuring with Florence's remnant at this time. The stronger/vertically deeper Florence becomes (assuming we have regeneration)...she could track even more north while feeling influences from mid-ocean upper vorticity in paragraph P3 and paragraph P1 upper trough emerging from the eastern US.

This tropical wave...located east of Florence's remnant and well WSW of the Cape Verde Islands...has been continuously organizing since it left Africa. Its t-storm driven latent heat release has successfully locally inflated the south side of paragraph P6 upper ridge such that upper easterly shear is reducing...and more of an upper anticyclonic outflow continues to be established. Its thunderstorms continue to be sufficient in keeping out paragraph P5 dry air mass. With this reasoning...I continue to expect this tropical wave to become our next Atlantic tropical cyclone. W Atlantic upper ridge (paragraph P1) is nearly vertically stacked with Atlantic surface ridge (paragraph P5) for deep-layered easterly flow very supportive of a continued west track. In the longer-term...we shall see how this system interacts with any ridge weaknesses of paragraph P1 upper trough/surface frontal system...which would help in a northward bend or turn at some point. Interests in the Lesser Antilles and northern Caribbean Islands should monitor this system carefully as it is unclear if it will track north enough to hit or miss these islands.

P1...Upper trough over central US and central Canada persists...and is spreading into eastern areas. First frontal cyclone just SW of Greenland has crossed Greenland and left the scope of above birdseye charts. Second frontal cyclone has swung from from Hudson Bay into waters just W of Greenland in the last 48 hrs...strengthening from 1002 to 998 mb in that timeframe. Central US surface ridge continued to weaken while being pushed into eastern US ahead of second frontal cyclone...joining the Atlantic ridge (paragraph P5) with a 1017 mb center over the SE US. But this central US surface ridge is re-building behind the second surface cyclone with support of upper convergence on back side of this upper trough regime...as seen with 1020 to 1021 mb centers in upper-left corner of above atmo birdseye chart. Low-level warm air advection ahead of second cyclone supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough regime.

P2...Longwave upper trough regime across Atlantic high seas has concentrated into the NE Atlantic. Surface frontal depression (cyclone) diving SE toward Azores has become stationary just north of those islands....its cool air adveciton wrapping around a good bulk of the longwave upper trough into a large upper vortex stacked over the surface cyclone. Whatever did not get wrapped above the surface cyclone is a cut-off upper low just north of the Cape Verde Islands.

P3...Mid-ocean upper vorticity persists....now featuring a couple of upper vortex centers...one entering the Caribbean Sea while retrograding SW about W Atlantic upper ridge (paragraph P1)...the second located well SE of Newfoundland.

P4...All cut-off upper vorticity in Gulf of Mexico has retrograded westward into Mexico....pushed vigorously by the strong upper anticyclonic outflow of Tropical Storm Ernesto.

P5...Atlantic surface ridge with a 1025 mb center is supported by a few upper convergent sources while stretching from Gulf of Mexico/SE US to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P1)...and convergence behind NE Atlantic upper vortex (paragraph P2). In conjunction with south side of W Atlantic upper ridge (paragraph P1)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air has been eradicated across the Caribbean Sea and Gulf of Mexico due to moisture from Ernesto...the remnants of Florence...and moisture produced by interaction of Caribbean upper vortex (paragraph P3) with a tropical wave (paragraph P7). Tropical wave Invest 92-L is helping to reduce this dry air in the eastern tropical Atlantic. Within this surface ridge...a surface trough E of Bahamas had been supported by by upper divergence between mid-ocean upper vorticity in paragraph P3 and W Atlantic upper ridge (paragraph P1). This surface trough is dissipating...what is left of it is SE of Bermuda at this hour.

P6...Upper ridge across Atlantic tropics has been split in half by paragraph P3 upper vorticity retrograded SW into the Caribbean Sea. West half is concentrated over Ernesto...providing great anticycloinc outflow over the storm as noted in above Ernesto special feature section. East half of this upper ridge is being locally inflamed into cells of upper anticyclonic outflow via t-storm latent heat release from the remnants of Florence and tropical wave Invest 92-L...with upper troughs forming between these cells. For instance...there are inverted upper trough signatures between Invest 92-L and Florence...located east of the Lesser Antilles.

P7...Tropical wave in eastern Caribbean Sea is interacting with divergent southwesterly flow on SE flank of Caribbean upper vortex (paragraph P3 feature). This has greatly increased t-storm activity in this region. There is also increased t-storm activity ahead of this tropical wave in the west Caribbean/Central America...supported by split flow upper divergence between this upper vortex and Ernesto's upper anticyclone. Despite all this t-storm activity...no tropical cyclone development is expected due to westerly vertical shear across the south side of this upper vortex.

P8...Satellite imagery over Africa shows a very impressive tropical wave about to enter the eastern tropical Atlantic waters...as marked in the lower-right corner of above atmo birdseye chart.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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